Hydrology, Cryosphere & Earth Surface News

Biggest Risk to Surface Water After a Wildfire? It’s Complicated

Whether you’re considering short-term or long-term changes to water quality after a wildfire, scientists agree that sedimentation is a big concern.


California’s CZU Lightning Complex Fire is 100% contained, but it’s still smoldering almost 2 months after it started and people were evacuated. For those residents of Santa Cruz County who returned to find their homes still standing, the sense of relief soon turned back to anxiety as they received mixed messages about how safe their water was.

Weeks after the fire was contained, some residents were still under “do not drink/do not boil” orders—unable to use their tap water to even bathe their children, said Hannah Hageman, a journalist living in Santa Cruz. Others for whom such orders have been lifted still don’t know whether to trust their water, Hageman said. “It’s been a traumatic couple of months for residents” of the coastal community.

Typically, the biggest concern for water managers and water researchers after a wildfire is sediment, as erosion contributes to sedimentation in surface water everywhere a wildfire burns.

After the Burn

In Santa Cruz County, however, benzene, a known carcinogen, and other volatile organic compounds (VOCs) were found in drinking water samples. VOCs and semivolatile organic compounds are especially worrisome, said Andrew Whelton, an environmental engineer at Purdue University in Indiana who coauthored a study in the July/August issue of AWWA Water Science. Whelton and his colleagues noted that benzene identified in Santa Rosa, Calif.’s water after the Tubbs Fire in 2017 and in Paradise, Calif., after the Camp Fire in 2018 was found in the water distribution network where plastic pipes and houses burned, not in the source water.

For source water, immediate concerns in the wake of a fire include microbes or bacteria like E. coli, mercury and other metals that might be volatilized by a fire and rained down or deposited by ash, ash itself, increased sediment, and disinfection by-product precursors. Disinfection by-products are compounds formed when water treatments like chlorine or other disinfectants react with dissolved organic matter (DOM) in water.

All of these water problems can arise with any wildfire, said Chauncey Anderson, a water quality specialist at the U.S. Geological Survey (USGS) Oregon Water Science Center. Each watershed will experience a discrete set of primary risks for a fire, he said. Topography, including slope, affects how much erosion occurs. The type of vegetation (or structures) burned and what was in the soil affect what ends up in surface water. The fire’s mechanics, like how hot the fire burned and how much of the watershed burned, affect what’s in the runoff. The hydrology of the area, the climate of the area, and how the watershed supplies people also matter, Anderson said. Thus, the exact “effects of these fires on drinking water are going to be highly variable.”

Sedimentation: A Wide-Reaching Threat

Sedimentation, however, is the first and arguably biggest threat to surface water supplies, Anderson said. By denuding landscapes of their erosion-controlling vegetation, wildfires leave them primed for erosion. When rains hit, tremendous amounts of ash and sediment wash into rivers and reservoirs, causing physical disruptions in two forms. In the first, large-scale runoff (in the form of landslides and debris flows) clogs intake pipes at water treatment facilities and fills reservoirs with sediment. In the second, smaller-scale runoff increases the amount of fine sediment suspended in water.

Sedimentation also decreases the water quality itself by changing the amount and type of DOM, said Alex Chow, a biogeochemist at Clemson University and coauthor of a study in Water Research about DOM, disinfection by-products, and nitrogen after fire. As more and different organic carbon is released into surface water after a fire, it is more difficult for municipalities to use conventional treatment processes without accidentally creating disinfection by-products. Nitrogen compounds are another issue, Chow said. Fire releases nitrogen stored in plants and trees. “In the first year after a fire, we mainly see ammonium. In the second year, we see more nitrates released,” he said. Chow’s research has shown that watersheds can be disrupted for up to 15 years after a fire.

Research also indicates that the presence of phosphorus and some metals may increase in water after a fire, said Charlie Alpers, a USGS research chemist. The 2015 Rocky and Jerusalem Fires in Northern California, for example, burned an area affected by historical mercury mining in Cache Creek. The fire caused mercury levels in burnt soils to decrease because the mercury was volatilized and transported elsewhere. But the increased quantity of suspended sediment for a given flow balanced the effect of lower mercury on suspended particles from eroded soil, he said; thus, the overall amount of particulate mercury transported downstream was similar to prefire levels.

Johanna Blake, a geochemist at the USGS New Mexico Water Science Center, has also found that different trees release different metals, in different concentrations, when burned. When one type of tree, say, a ponderosa pine, burns, it releases whatever metals it has absorbed from the soil and air, like iron and manganese. When another type of tree, like an aspen or spruce, burns, it might emit more vanadium, lead, magnesium, or copper, Blake said. “But we’ve barely scratched the surface” on questions like “what’s going to cause a [metal’s] release from the ash and what kind of geochemical reactions the ash and sediment will cause in the water.”

Sedimentation is also dependent on precipitation. For instance, Alpers said, the first year after the Rocky Fire was pretty dry, so the effect of higher than usual suspended sediment concentration (for a given flow) persisted in the river for more than a year. If there’s a drought, he said, the rain is not there to flush out the system. On the other hand, if there’s a lot of rain, sedimentation may be extremely high immediately but taper off quickly, said Kevin Bladon, a forest hydrologist at Oregon State University.

If It Burns, It Will Run Off

Because whatever is in the sediment of a burned area will likely end up in surface water—whether it’s contaminants from paints and pesticides or ash from vegetation—scientists and water managers want to know what is in the sediment. Alpers and Anderson, among others, are often in the field sampling soils and water even before fires are completely extinguished.

After the Holiday Farm Fire in Oregon this fall, Anderson and his USGS colleagues scrambled to install a new stream gauge and real-time water quality monitoring station on the McKenzie River after the one upstream of water treatment facilities burned. “We talked to our partner agencies, to water quality managers, to find out what they needed: They needed basic infrastructure back up and running so they could see what’s coming in real time,” Anderson said.

In California, Alpers and his USGS colleagues sampled ash and soils near Lake Berryessa, a surface water reservoir whose watershed burned during the LNU Lighting Complex Fire. Over a gradient of burn severity, the team is evaluating mercury, nitrogen, phosphorus, and carbon content. They are working with the Bureau of Reclamation to use these constituents to build a model for water managers to use after a fire. The model would ideally let water managers know “here’s how long you can expect water quality to be different, how long will it take to recover depending on how many storms and what magnitude they are, and how much sediment’s going to come into your reservoir,” Alpers said.

Burned landscape near Lake Curry, Calif.
USGS teams sampled ash and soils near Lake Curry, which burned during the LNU Lighting Complex Fire in August and September 2020. Credit: USGS

“There’s a new push to do more research related to wildfires and water quality—especially after this year,” Blake noted. Part of the reason for that is that most of the postfire water quality research is based on data from California fires, Bladon said. But a place like Oregon has a vastly different precipitation regime, plus different soils and geology, than California or elsewhere, Bladon said. So it’s important to capture as much data from Oregon—and everywhere—as possible. After all, he said, these fires are going to keep happening.

And, Anderson said, they are leaving lasting effects: “In Oregon, we haven’t had many prior situations where significant drinking water supplies were so seriously and directly affected by wildfire,” he said. “The overall effect is that a very large percentage of Oregon’s population is likely going to have [long-term] effects that range from increased treatment costs to potential health effects.”

Megan Sever (@MeganSever4), Science Writer

Citation: Sever, M. (2020), Biggest risk to surface water after a wildfire? It’s complicated, Eos, 101, https://doi.org/10.1029/2020EO150691. Published on 20 October 2020.
Text © 2020. The authors. CC BY-NC-ND 3.0
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